1. Field of the Invention
The present invention relates to a gas sensor having a protector for preventing water droplets present in exhaust from adhering to a detecting element exposed to exhaust gas.
2. Description of the Related Art
Conventionally, a gas sensor is known having a detecting element which generates an electromotive force having a magnitude corresponding to the concentration of a specific gas; for example, NOx (nitrogen oxides) or oxygen, contained in exhaust gas of an automobile or the like or whose resistance varies with concentration. The gas sensor is attached, for use, to an exhaust pipe of an automobile or the like. Since the detecting element is exposed to exhaust gas of high temperature, subjecting the detecting element to thermal shock caused by adhering water droplets contained in exhaust gas may crack or break the detecting element. In order to cope with this problem, the gas sensor is provided with a protector for covering the detecting element, thereby protecting the detecting element from adhering water droplets (refer to, for example, Patent Document 1).
The protector of the gas sensor of Patent Document 1 has a dual structure consisting of an outer tubular member and an inner tubular member. A front end portion (a lower portion when the gas sensor is attached) of the outer tubular member has introduction holes for introducing exhaust gas toward a clearance between the outer tubular member and the inner tubular member (hereinafter, also referred to as a “gas separation chamber”). Guide portions are provided at respective introduction holes of the outer tubular member for causing exhaust gas introduced into the gas separation chamber to swirl around the outer circumferential surface of a side wall of the inner tubular member. Inertial force associated with the swirl separates a gas component and water, which is heavy relative to the gas component, from one another. A rear end portion (an upper portion when the gas sensor is attached) of the inner tubular member has introduction holes for introducing the gas component, which is separated in the gas separation chamber, into the interior of the inner tubular member.
[Patent Document 1] Japanese Patent Application Laid-Open (kokai) No. 2004-125702
3. Problems to be Solved by the Invention
However, even when a gas component and water are separated from one another as in the case of Patent Document 1, water, together with the gas component, may enter the interior of the inner tubular member. This depends on the velocity of exhaust gas introduced into the interior of the outer tubular member, the mass of the water droplet, the position of the gas sensor attached to an exhaust pipe, and other factors. Adhesion of a large water droplet to the detecting element may impart thermal shock to the detecting element to a degree so as to crack or break the detecting element.